So how I interpreted it is.The nature of time is such that, once the sacred timeline is broken, it always was broken. The break happens "outside time", such that branching happen both before and after Loki stepped out of the timeline and into the TVA. In effect, throughout the MCU, the multiverse has always existed. The Ancient one (who still has the Time Stone) was the only one we'd seen aware of the multiverse. The Loki series tells us why, for us viewers who are also "outside time", the multiverse was never mentioned before (where "before" is for us outside the timeline).
How's that for a No Prize explanation?
I don't believe that's ever clearly and unequivocally stated. It could just as easily be that any variance under a certain amount will ultimately rejoin the main timeline eventually. If it doesn't reach an "escape threshold", they can ignore it, as it comes back. Or maybe there's another explanation for their way of dealing with some variances and not others.
The Multiverse is like a giant ball, and the various "dimensions" float around it (like the dark dimension). So even though there are many universes, there is only one dark dimension.
This is why that Shu-Gorath wannabe in MoM was able to follow America through the various universes, to him each one is just a different connection point from his dimension. The multiverse is all just "one big place" to dimension hoppers.
So in the original Doctor Strange, the dimensions they talk about aren't really "multiverses", again this is the stuff that's outside the multiverse reality. Its in Loki that the one universe (surrounded by these various dimensions) gets split back into teh multiverse again.
That said, it honestly is quite confusing how they have portrayed it
